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Abstract

A revision was undertaken of the lithostratigraphy of the Rogaland Group for the Norwegian
North Sea. An abundance of recent well and seismic data sheds new light on lithology,
biostratigraphy, provenance and geographic distribution of all Rogaland units.

Whilst finer siliciclastic units largely remain as previously defined, sandstone/siltstone
formations and one (reworked) chalky unit are now included as members. With the new
definitions and redefinitions the Rogaland Group now consists of four formations and
seventeen members (Table 1), which span the stratigraphic interval from lower Paleocene to
lower Eocene. The revisions concerning the sandstone bodies are of four different types:

Redefinition from formations to members

Redefinition of lithological criterias

Introduction of members used in UK and DK

Definition of new members

Table 1. Overview of the Formations and Members of the Rogaland Group, Paleocene - Lower
Eocene, North Sea, Norway.

1. GEOLOGICAL SETTING FOR THE ROGALAND GROUP, NORWEGIAN NORTH SEA.

1.1 General setting

Fig. 1. Structural elements of the Norwegian North Sea at BCU level, with the first
Paleogene hydrocarbon discoveries in Norway (Balder, 1967), UK (Arbroath, 1969) and Denmark
(Siri, 1996) outlined.

Paleocene to lowermost Eocene strata are extensively distributed throughout the whole of
the North Sea Basin, and provide one of the most prolific hydrocarbon plays in the
Norwegian North Sea. Hydrocarbons were first discovered in strata belonging to this age in
1967 at the Balder Field
(Fig. 1)
in the Norwegian sector and the first discovery in the
UK, the Arbroath Field was in 1969. In the Danish sector, the Siri Field discovery was made
in 1996 (Ahmadi et al., 2003).

Paleocene to lowermost Eocene sediments in the central and northern North Sea range in
depths close to sea bed to more than 3000m, and reach a thickness of approximately
1000 m
in the Outer Moray Firth.
Fig. 2 shows a time depth map and areas of shallow and deep
burial, whereas the time thickness map in Fig. 3 shows distribution of thin and thick
development of the Rogaland group.

Improvements in seismic acquisition and processing quality and the application of high
resolution biostratigraphy have caused a sustained interest in this stratigraphic interval.
Although this play model has existed for a long time, discoveries are still being made,
exemplified by the 16/4-4 and the Storskrymten discoveries made in 2007.

The Rogaland Group comprises the most sandprone interval of the Paleogene (Ahmadi et al,
2003; Liu & Galloway, 1997). The preserved strata in the North Sea consist of
siliciclastic sediments with volumetrically minor amounts of coal, tuff, volcaniclastic
rocks, marls and reworked carbonate sediments. The Scotland-Shetland hinterland was the
primary source for the great volumes of siliciclastic sediment (Morton et al., 1993; Ahmadi
et al., 2003), but also Fennoscandia acted as a provenance.

Tectonic activity at this time was related to the development of the Iceland Plume, which
caused regional uplift and gradual enclosing and isolation of the North Sea basin from the
Atlantic Ocean
(Fig. 4)
. Adjacent to the North Sea, the Scottish Highlands and the East
Shetland Platform were uplifted, together with a somewhat less magnitude of uplift of
Western Fennoscandia. Further stresses along the line of the future north-east North
Atlantic Ocean led to major volcanic activity in that region, which is represented in the
sedimentary strata as tuffs.
The seismic time thickness map in Fig. 3 clearly demonstrates
the development of thick Paleocene-Earliest Eocene deposits adjacent to these areas.

Fig. 4. Global reconstruction of North West Europe in the Late Thanetian, the period of
maximum basin restriction of the Early Paleogene. Modified from Ziegler (1988), Torsvik et
al. (2002) and Coward et al. (2003).

1.2 Basin development in North Sea - Proto Norwegian Sea Basin, a semi-enclosed to
enclosed basin system

A short summary of the basin evolution is given below
and in Fig. 6
.
For reference in the
following text we include the 2004 Geological time scale (Fig. 5).

Fig. 5. Geological Time Scale 2004 (Gradstein et al., 2004, modified by Simmons et al,
2007) highlighting stages with Global Stratotype Selection Points (GSSP), which is a
location specific bedding plane where the base of each stage is defined. This definition is
tied to an event in the rock record useful for correlation. As can be seen one GSSP has
been defined within the time period when the Rogaland Group was deposited, the
Thanetian/Ypresian transition 55.8Ma, which is internally in the Sele Formation, close to
the boundary between the Upper and Lower Sele Formation.

Fig. 6. Simplified basin development in the North Sea Basin through the Early Paleogene.

Early Danian

The warm, well oxygenated marine environment that existed in the North Sea during the Late
Cretaceous continued into the Danian. Southern and Central parts of the North Sea were
depositional sites remote to siliciclastic input, and sediments were dominated by
calcareous, coccolithic mudstones. In the northern North Sea however, shorter
distance to
terrestrial provenance areas caused more siliciclastic sedimentation in that area than
further south.

Late Danian-Early Selandian, increased siliciclastic input (Våle Fm)

The first pulse of the Alpine orogeny (Cretaceous to Early Paleogene, e.g. Ebner, 2002)
affected the southern and central parts of the North Sea with inversion tectonics and local
movements along some of the deeper fault lineaments inherited from pre-Cretaceous
times.
This caused tectonic uplift and sea level drop with exposure of more terrestrial areas
closer to the North Sea Basin. There was also a change to a more temperate and more humid
environment. With increased siliciclastic input and less favourable temperature the
coccolithic carbonate systems of the Chalk Group were gradually switched off, and
deposition of the Rogaland Group started.

In this period the calcareous input from microorganisms, and reworking from exposed chalk
of the Shetland Group had practically come to an end, and the fines deposited in the
basin
were dominated by siliciclastic minerals. In general the trace fossils became smaller in
size and the sediments became darker, reflecting less oxygenation of the basin in this
period. Coarser sediments were reworked and redeposited during three major episodes of sea
level drop in this period.

Tectonics seem to have caused basin restriction due to establishment of threshold barriers
between the North Sea and the Atlantic Ocean. The thresholds were established by
inversion
tectonics in the London - Brabant Platform area in the southwest, rise of the Wyville
Thompson Ridge west of Shetland, and early plate collisions in the south and east,
isolating the North Sea from the Tethys Ocean and the Arctic Oceans. As a consequence the
North Sea basin became dysoxic to anoxic for a period that lasted for 3 million years, and
thus well into the Early Ypresian. Coarser sediments were introduced into the basin during
two major episodes of sea level drop.

Early Ypresian (Balder Fm)

The basin isolation and restriction during the Late Thanetian continued into the
Early
Ypresian. During this period the basin became strongly influenced by tuffaceous ash
falls related to magmatism and extrusive activity associated with the breaking up of the
North Atlantic. Sandy sediments were introduced into the North Sea Basin during a sea level
drop in this period.

Middle to Late Ypresian

In the Middle to Late Ypresian the North Sea Basin and the connection to the Atlantic and
possibly the Tethys oceans were temporarily reestablished, and the basin became better
circulated. Benthic fauna fossils were reestablished and light green grey to red
colored, heavily burrowed mudstones were deposited within the lowermost part of the
Hordaland Group.

1.3. Some stratigraphic markers of regional importance

The boundary between the Shetland Group and the Rogaland Group is regionally the most
important unconformity affecting the Rogaland Group. It is extensively developed and is
usually a very good lithostratigraphical marker, making the distinction between the
Rogaland and Shetland Groups easy in most cases
(Fig. 7)
. However, the unconformity is
diachronous in many areas and can hence not be used as a high resolution
chronostratigraphic marker.

Fig. 7. Showing the unconformity at the base of the Rogaland Group, with the Våle formation
resting on the chalk of the Shetland Group. Example is taken from Well 25/11-17
drilled by
Norsk Hydro. Photo from NPD faktasider at http://www.npd.no.

Within the stratigraphic interval spanning the Sele Formation there are two important
unconformities in the southern North Sea and the English Channel - London Basin area
(Fig. 8)
. Of these two the most important and easiest to recognize is the one represented by the
Lista-Sele facies transition (Latest Thanetian-Sparnacian) which reflects a dramatic sea
level fall, as seen throughout the UK shelf and onshore, and a simultaneous basin
restriction
with anoxia in central and deeper parts of the North Sea basin.

Fig. 8. Chronostratigraphic chart, modified after Knox (1994) for the Paleogene of the
North Sea-Paris Basin region, showing that the type areas of the Paris Basin, the
Ypresian
and Thanetian stages, are bounded by unconformities. To capture the missing time
represented
by these unconformities, the stages could be expanded downward to incorporate the time
represented by the basinal sediments of the Central and Northern North Sea Basin where
there is continuous deposition. Modified from Simmons et al. (2007).

The overall fall in sea level for Southern England is inferred as being at least 100
m.
In Bradwell (Knox et al. 1994), the results of this sea level drop can be seen as an
interval of fine grained marine mudstones of the Rhabdamina biofacies (Thanet Formation,
Lista equivalent), overlain by the pedogenically altered continental to lagoonal
Reading
Formation, Sele Fm equivalent. At the Isle of Wight, offshore South England
(Fig. 9)
, the
pedogenically influenced strata of the Reading Formation are seen to lie unconformably
between marine sediments of the Shetland Group and the London Clay Formation (time
equivalent to the Balder Formation). Knox (1996) and Neal (1996) argue that a sea level
fall of a magnitude and rapidity to have caused this effect must be related to tectonical
uplift. Ziegler (1988) and Ahmadi et al. (2003) infer that inversion tectonics has
influenced the Southern and Central parts of the North Sea Basin, whereas e.g. Nilsen et
al. (2005) also demonstrate substantial inversion along the Tornquist line and in the
Norwegian Danish Basin during the Paleocene. From France and Belgium there is documented
inversion of the Brabant Massif and the Artois Axis (Vandenberghe et al. 1998).
For reference, see Fig. 4.

Fig. 9. Exposure of the Reading Formattion at Whitecliff Bay, Isle of Wight. As can be
seen, the terrestrial sediments of the Reading Formation can lie inconformably between
marine sediments, and must represent a dramatic sea level drop relative to the adjacent
sediments (Photo by H. Brunstad).

According to Simmons et al. (2007) the Sele-Lista Formation interboundary is present
as an
unconformity as far as the southern part of the Central North Sea
(Fig. 8)
, and Dreyer et
al. (2004) report shallow marine indicators within the overall deep marine sandstones of
the Latest Thanetian Forties Member in the Pierce and the Josephine High areas. These
shallow marine indicators could be a result of local tectonics/salt tectonics, but could
also as well be explained as a result of more regional causes, reflecting the large scale
inversion related sea level drop as mentioned above.

In the sediments from the Central and Northern North Sea can be seen an abrupt
transition from bioturbated to practically undisturbed dark shales. This transition is due
to the above mentioned anoxia, lasting through deposition of the entire Sele and Balder
Formations, ending with the deposition of the green and red coloured bioturbated shales of
the Hordaland Group. Both top and base boundaries of this anoxic shale zone are sharp and
considered to be very good litho- and chronostratigraphic markers. In the Central to
Northern North Sea the basal part of this zone represents the correlative conformity to
the subregional unconformity seen at the base of the Reading Formation in the London Basin
and at the Isle of Wight, and in the South to Central North Sea.

The Sele-Lista Formation interboundary thus seems to represent an unconformity in the
Southern
North Sea and along the east and west margins of the basin further north, and a correlative
conformity in the basinal areas from the Central North Sea and northwards. This
interboundary
is one of the best regional stratigraphic markers, easily detectable from seismic,
lithology and biostratigraphy.

Superimposed on the effects of this period of basin isolation there was also a global
warming pulse
(Fig. 10)
, the Paleocene-Eocene Thermal maximum (PETM) with global
extinction of marine benthos outside the North Sea Basin. This pulse raised the global
oceanic temperature by 6-7°C for a period of ~60.000 years (~54.98-54.92Ma) by the
beginning of the Ypresian (Bowen et al. 2004). This short term effect is difficult to
distinguish from other parts of the Sele Formation, since the basin was already anoxic. Sea
surface temperatures rose between 5 and 8°C over a period of a few thousand years. Although
the extreme PETM warming lasted only for a short period, global sea surface temperatures of
the Paleocene-Early Eocene period before and after this event were still much warmer than
today. The Arctic and Antarctic seas were rather warm, and ice free (Bowen et al., 2004 and
Moryan et al. 2006).

With its short time duration, the PETM is almost a perfect chronostratigraphic marker.
However, the definition of this stratigraphic event is not possible macroscopically, and
must rely on high resolution biostratigraphic analysis with dense sampling.

Volcanic extrusives regional seismic marker

The igneous activity in the North Atlantic
(Fig. 4)
shows a wide age range, but peaks
between 55 and 50 Ma (Torsvik et al, 2002), spanning syn rift and a continental break-up
phase. During the late rift phase, and especially during deposition of the Balder
Formation
(~53-54 Ma), large amounts of tuffaceous ash material were introduced into the
atmosphere,
and distributed over vast areas of North Europe. The lower and upper boundary of the tuff
rich zone
(Fig. 11)
is a significant litho- and chronostratigraphic marker that makes
recognition of the base of the Balder Formation rather easy in most cases. Some minor
tuff
stringers are also sometimes seen in the Lista and Sele Formations, but are not easy to use
for correlation.

Fig.11 Series of bright to medium dark greenish grey tuff layers of variable thickness,
interlaminated with black anoxic shales. Example is taken from the Balder Formation Well
25/11-20 drilled by Norsk Hydro. Picture from NPD Faktaside at
http://www.npd.no.

2. LITHOSTRATIGRAPHIC NOMENCLATURE

2.1 General

This study deals with a revision and update of the lithostratigraphic nomenclature and
classification of the Rogaland Group of Paleogene age in the Norwegian sector of the North
Sea below 62°N.

In broad terms, the Rogaland Group of this study corresponds with the formal definition by
Hardt et al. in Isaksen & Tonstad (1989), which authors joined Deegan & Scull's (1977)
Montrose and Moray Groups into the single Rogaland Group.

In this study, the finer siliciclastic formation units remain as previously defined, but
the sandstone formations of Hardt et al. (1989) are changed into members. This procedure
follows much of the same principle as in Mudge & Copestake (1992) and Knox & Holloway
(1992) for the UK sector of the North Sea.

Since Hardt et al.'s (1989) work on the Norwegian sector, an abundance of new stratigraphic
information from wells and 3D seismic has become available. The data have shown a more
complex situation concerning sand provenance and sand distribution. The geographic
distribution of the sand units has been refined. The stratigraphic resolution using
microfossils has also become more detailed.

As a result, several new member names have been introduced in the period after Hardt et
al.'s (1989) revision (Knox & Holloway, 1992; Ahmadi et al., 2003; Schiøler et al., 2007)
and are used here. In addition, new members are proposed in this study, and several type
and reference wells originally defined by Hardt et al. (1989) have been amended. Several
additional wells are also added when new stratigraphic units are established. This study
also provides comprehensive geological figures of the new members.

Member is a formal lithostratigraphic unit, next in rank below formation, possessing
lithologic properties distinguishing it from adjacent parts of the formation. No fixed
standard is required for the extent and thickness of a member. A formation need not be
divided into members unless a useful purpose is served. Specially shaped forms of members
(or of formations) are lenses and tongues. A member can transect in more than one
formation.

In the current revision of the Rogaland Group, it is intended to use existing naming as far
as practical for the sandstone members. However, some major additions have been made in the
North Eastern North Sea and in the Siri Canyon of the southern parts of the Norwegian
Danish Basin since the former official nomenclature (Hardt et al., 1989) did not describe
any sandstones there. Since the 90s several wells have given more information about
sandstones in these areas and they are correspondingly included and defined here.

The fifteen sandstone members of the Rogaland Group have been defined according to their
provenance area, sedimentary distribution and structural boundaries. This practice has been
used by: Hardt et al. (1989) to separate their easterly derived 'Fiskebank
Formation' from
the westerly sourced 'Forties Formation'; Schiøler et al. (2007) to separate easterly
sourced
sandstones of the Siri Canyon and Tail End Graben/Søgne Graben from westerly sourced
sandstones
of the Central Graben; and Hardt et al. (1989) and Knox and Holloway (1992), distinguishing
northwestern from southwestern sandstones in the Central Graben and the Viking Graben.

Roughly, sandstone members within each shale formation are bounded by the two crossing
Member separation lines shown in Figs. 12 and 13, which give four sub areas each with
their separate sandstone member:

North-South separation line.

West-East separation line.

Fig. 12. Sketch showing rough distribution of the sandstone members belonging to the four
shale formations of the Rogaland Group, relative to the Member separation lines.

Fig. 13. Sketch demonstrating subareas and separation lines used to limit the various
sandstone members.

2.3 Subjects covered in Lithostratigraphic nomenclature

In this revision each of the four formations and the fifteen members are described in the
same successive steps:

Unit definition

Name

Derivatio nominis

Type well

Reference wells

Composition

Wire line log characterization

Thickness

Seismic characterization

Age

Biostratigraphy

Correlation and sub division

Geographic distribution

Depositional environment

The amount of text dedicated to each formation or member is dependent on the data coverage
and importance.

In general the shale formations are described in more detail concerning biostratigraphic
events and stratigraphic criterias, and in the descriptions of the sandstones members it
is often referred back to the shale formation concerning this issue.

A large amount of composite well diagrams have been designed, and are shown under the
various stratigraphic units.
The lithological codes used are shown in Fig. 14.

Fig. 14. Lithological codes used in well composites in Chapter 4-7.

3. Rogaland Group, general description

Unit Definition of the Rogaland Group

The Rogaland Group of the Norwegian sector of the North Sea corresponds to the combined
Montrose and Moray Groups of the UK sector (Knox & Holloway, 1992). It is divided into
four shale/mudstone formations. From old to young these are the
Våle, Lista, Sele and
Balder Formations, each containing their respective
sandstone members (Table 1
and Fig. 15
).

The formations in this lithostratigraphic revision are defined geographically to include
the main shale units of the Paleocene and Early Eocene of the North Sea up to 62°N.

An overview of the Rogaland Group with its four shale formations and seventeen
sandstone members is shown in Fig. 15.

Name

The Rogaland Group was given name by Deegan and Scull (1977) to Paleocene to Early Eocene
(Late Danian-Early Ypresian) siliciclastic sediments of the Central and Northern North Nea.

Derivatio Nominis

The name Rogaland is after the county of Rogaland in southwest Norway.

Lower Boundary

In the central North Sea, the Norwegian-Danish Basin and in the southern Viking Graben the
base of the Rogaland Group is picked at the change from marly mudstones with local
sandstones and thin interbeds of limestone of the lower parts of the Våle Formation into
the chalks and marls of the Shetland Group. In the
northern North Sea the basal boundary is
not so cleary defined as further south, since there is less distinct lithological
difference between the Våle Formation and a less calcareous underlying Shetland Group
in that area compared to further south.

Upper boundary

The top of the Rogaland Group is taken at the transition between the dark grey,
laminated
and partly tuffaceous shales of the Balder Formation into
the commonly pale green grey to
red colored basal parts of the overlying Hordaland Group.

General lithological characterization

The Rogaland Group is characterised by a siliciclastic succession of sediments following
the more carbonate rich Cretaceous Shetland Group. The sediments are composed of basin-wide
mudstones and shales, with intercalations of sandstones at several levels that vary widely
in geographical distribution.

Wireline log characterization

From wire line logs (Table 2) the Rogaland Group is defined by a significant upwards
increase in gamma readings and a decrease in acoustic velocity when compared to the chalky
or calcareous mudstones of the Shetland Group. The top of the Rogaland Group is often taken
at or close to the top of the bell shape seen on wireline logs for the Balder Formation.

Table 2. High resolution shale stratigraphy of an idealized desanded log of the Rogaland
Group in the Viking Graben.

Thickness

The thickness of the Rogaland Group varies significantly within the North Sea Basin. In
the Norwegian sector a thickness of 918m was found in well 24/9-3, 714 m in 25/4-3,
and 647 m
in well 35/9-2. Very minor thicknesses of the Rogaland Group are seen on top of salt
diapirs in the Central Trough (eg. 26 m in well 1/6-5), and on the Utsira High (88 m
in well
16/6-1), and in the easternmost parts of the Norwegian Danish Basin (55 m in well
9/2-2).

Seismic characterization

A tectonic map of the Central and Northern North Sea with the main structural elements is
shown in Fig. 1 and regional seismic maps from the Rogaland Group in Figs. 2 and 3.
These show an axial deep close to the UK/Norwegian border, with its deeper parts in the
south. Greatest thicknesses are seen in the southern Viking Graben Area and along the
eastern margin of the Sogn Graben.

The top of the Rogaland Group is often distinguishable as the top Balder seismic reflector.
However, in some areas the top of the Balder tuffaceous zone or the base of the Balder
Formation are better defined than the formation top, and is often easier to map regionally.

Regional Cross Sections

From regional seismic cross sections
(Figs. 16-18)
, prominent progradational wedges with
clinoforms can be seen west of the main depositional troughs of the Central Graben and
Viking Graben. Along the eastern margin of the Stord Basin and the Sogn Graben similar
wedges can be seen to dip from the East. The general impression is that clinoforms along
the western flank have a steeper dip than those from the east. This may be
interpreted as
the result of different sand to mud ratio in the sediments, and to a more active tectonic
regime of the Shetland Platform than in southern Fennoscandia. During the Paleocene
there
were periods of active rifting along the western margin of the Shetland Platform which
uplifted the platform and generated a more immature topography exposing erodable sediments,
whereas southern Fennoscandia to the east remained a tectonically stable platform with a
rather mature topographic profile in a landscape dominated by basement rocks.

Fig. 16. Regional seismic line I, northern North Sea. Note signs of dipping
reflections/shelf buildout in the East Shetland basin, and dipping
reflections representing slope wedges in the Horda Platform area. In the
basinal area between the wedges there are thin deposits, reflecting a
relatively sparse input of gravity flow material in that area compared to the
South Viking Graben and the Central Graben.

Fig. 17. Regional seismic line II, from the Shetland Platform to the Stord
Basin, North Sea. Dipping reflections east of the Stord Basin are attributed
to slope progradation from the East. The Viking Graben formed a deep basin
filled with a mixture of gravity flow sediments sourced from the East
Shetland Platform and background hemipelagic siliciclastics. Utsira High is
characterized by a thinning of the sequence, and is believed to have formed a
submarine basinal high during deposition.

Fig. 18. Regional seismic line III, through the Central Graben and the
Norwegian Danish Basin, North Sea. Thickening in the Central Graben is mainly
attributed to input of siliciclastics from the East Shetland Platform and the
Moray Firth area in the north, but input from Southern Fennoscandia in the
east and possibly the Midland Valley area may also have contributed. The
Central Graben area is commonly interpreted as a deep marine environment,
whereas the Norwegian Danish Basin is considered as a shallow to deep marine
transition.

Seismic character on a local scale

Sandstones are present in all of the fine-clastic formations. Top and base of
sand packages are often mappable due to contrast between blocky sandstones and
adjacent shales. In other cases the tops of the sandstones have a more gradual
transition to the shales above and there is not a good seismic contrast. In
these cases presence of sand is often seen to correspond to a mounded seismic
character with discontinuous internal reflectors.
An example of
seismic character of sandy facies is shown in Fig. 19.
Mounds are often
interpreted as containing submarine channels/channel complexes or elongated
submarine fan systems. In many instances seismic attribute analysis can
delineate the areal extension of these systems.

Fig. 19. Example of seismic character of sandy facies of the Rogaland Group,
seen from a WE section through southern parts of block 25/10.

Sediment composition and processes

Sandstone facies

In the Norwegian North Sea the Rogaland Group contains a wide variety of
sandstone facies, ranging from deep marine and gravity flow to shallow marine
and deltaic sandstones. Deep marine sandstones are dominating, but shallow
marine deposits have also been recorded. In general shallow marine facies are
found in the easternmost parts of the Norwegian North Sea, but at some stages
possibly also in parts of the southern Central Graben sucession (Dreyer et al,
2004).

The most pronounced deep marine sandstones are found along the trends of the
Central Trough - Viking Graben and the Sogn Graben. On the East Shetland
platform (UK), sandstones intercalated with shales and well developed coals or
lignitic beds witness deltaic/coastal plain setting. Distinct coal beds are
not found in wells penetrating the Rogaland Group of the Norwegian North Sea.

Most wells in the Norwegian sector have been drilled in a paleo-slope to basin
floor setting and sandstones here are in general interpreted as gravity
flow/turbidites of submarine fans (e.g. Hardt et al., 1989, Knox & Holloway,
1992). Possible exceptions include some occurrences of the intra Sele
Formation sandstones of the Forties (Dreyer et al., 2004) and Fiskebank
Members. This is discussed in more detail in the subchapters on these members.
A nice seismic scale example of submarine fan deposits is shown in the seismic
amplitude map in Fig. 20. Examples of turbidites in cores from the Rogaland
Group are shown in Figs. 21-24.

Fig. 20. Seismic amplitude maps of an intra Sele Formation seismic marker,
displaying submarine fan deposits of the Hermod Member in north eastern parts
of Q25.

There is a great variability in the composition of the turbidites. Some
consist of thick beds of massive sandstones with frequent fluid escape
structures and only very sparse primary structures
(Figs. 21 and 22)
. In other
cases turbidites consist of interbedded shales and sandstone layers with
primary structures as horizontal lamination and ripple cross bedding
(Figs. 23 and 24)
.

In general, the sandstones have very fine to medium sized grains and are
moderate to well sorted. Mineralogy is dominated by quartz and feldspar,
although in some cases the sandstones may contain a large proportion of
glauconite or mica grains. Rip-up clast material of clay and drifted wood
fragments may be common in the sandstones in some areas, especially in lower
parts of sandstone intervals.

Sandy debris flow deposits may also make up an important part of the primary
deposited sandstones in some areas, consisting of massive, rather
structureless sandstones, with variable content of fine grained clayey matrix
and larger angular mudstone clasts. In the sandstones of the Maureen and Mey
Members of the Central Trough, and the Ty Member of the Viking Graben, chalk
clasts and occasional chert clasts are also common within the sands.
An example of sandy debris flow deposits is shown in Fig. 25.

Fig. 25. Example of sandy/gravelly debris flow deposits from the intra Lista
Formation Mey Member. Cores are from well UK 30/14-1, Flyndre discovery, close
to the UK/NOR border. The clast material consists of reworked chalk,
mudstone, chert and sandstone. The sediments are matrix supported to grain
supported. Photo by H. Brunstad.

Injectites

One of the most prominent post depositional features of sandstones in the
Rogaland Group is the frequently occurring sand injection structures (e.g. De
Boer et al. 2007, Huuse et al. 2004). In the Rogaland Group such features are
observed on several scales from seismic
(Fig. 26)
to well core scale
(Figs. 28 and 29)
. In the cores the injectites are often seen to be of different
generations with networks crosscutting and connecting each other. This
indicates that the sand injection process occurred at repeated stages through
burial.

Sand intrusion and extrusion is a result of pressure release and pressure
reduction from overpressured sands through water escape and escape of
fluidized sands from higher pressured areas to lower pressured areas. Most
commonly this involved stratigraphical upwards movements of sands, but
downwards and lateral movements also seem to have occurred. Simultaneously
with sand injection there was severe sand deformation of parent and receiver
sand bodies and clay clasts being ripped up from sand/mud interboundaries,
both within the mother and the offspring sand bodies as well as in the sill/dike
systems themselves.

Genesis of overpressure

Sand mobilization and injection is believed to have occurred before
lithification, which in most cases is believed to have happened before several
hundreds of meters of burial. The system may have lived through several phases
of overpressuring with several different sources of overpressure. The most
relevant sources of overpressuring are considered to be

The most powerful and volumetrically most important source is believed to be
3., causing escape of high pressures from deeper basin levels created by
hydrocarbon generation and silica diagenesis.

Polygonal fault patterns

Polygonal fault patterns are commonly associated with the Rogaland Group. Such
patterns are especially well developed and frequently occurring in the Balder
Formation, but they are also common at other levels.
An example of this phenomenon is shown in Fig. 30.

According to Dewhurst et al., (1999) the structure and geometry of the fault
system are controlled by the colloidal nature of the sediments, and the
volumetric contraction measured on seismic scale can be accounted for by
syneresis of colloidal smectitic gels during early compaction.

Syneresis results from the spontaneous contraction of a sedimentary gel
without evaporation of the constituent pore fluid. This process occurs due to
the domination of interparticle attractive forces in marine clays, dependent
on environment, and is governed by the change of gel permeability and
viscosity with progressive compaction. The process of syneresis can account
for a number of structural features observed within the fault systems, such as
tiers of faults, the location of maximum fault throw and growth components at
upper fault tips (Dewhurst et al., 1999).

Slides and slumps

In some areas chaotic folds associated with sub horizontal shear planes
through the sediments are seen from drill cores, and are evidence of slumping
and sliding of sediments
(Fig. 31)
. Slumping is seen to be developed in
association with movements on slopes, but also in connection with slide
movements away from growing salt diapirs.

Fig. 31. Example of slump folded sediments. Example is taken from the Lista
Formation from well UK30/14-1, Flyndre Discovery, close to the UK/NOR border.
Photography by H. Brunstad.

Wireline log correlations

A series of lithostratigraphic wireline log correlations have been made at
four steps from north to south in the Norwegian North Sea
(Figs. 32-37)
.

The base of the Rogaland Group coincides closely with the last occurrence of
Senoniasphaera inornata (Mudge & Bujak, 1996). It is marked by a
reduction in
diversity of benthic assemblages towards the underlying Shetland Group (Knox &
Holloway, 1992). The age of the basal Rogaland Group is Late Danian and
approximately 61 Ma.

Table 3. Some important biostratigraphic events of the Rogaland Group.

Correlation and subdivision

Segmentation of the Paleocene-lowermost Eocene into stratigraphic sequences
was initiated by Stewart (1987), and has been continuously refined to date.
Since the work of Knox and Holloway (1992), lithostratigraphic subdivision of
this stratigraphic interval follows sequence stratigraphic boundaries. This is
also the case for this study.
The Paleocene succession in the North Sea contains two types of stratigraphic
surfaces (e.g. Mudge & Bujak, 1996 and Mudge & Jones, 2004):

Unconformity surfaces overlain by sandstones and reworked chalk or tuff
and representing submarine or sub aerial erosion and missing section.

According to Mudge & Jones (2004), biostratigraphic dating allows these
surfaces to be correlated throughout the North Sea Basin, as far south as the
Mid North Sea High. These authors found a series of short duration (0.1-0.3
my) unconformity-mfs couplets that may be recognized within the Danian (including
the Ekofisk Formation) to lowest Ypresian interval (65-53 Ma). These
uplift-subsidence cycles may have been caused by episodic plume related
magmatic injection near Moho and associated fluctuations in dynamic support,
related to the initiation of the Iceland Plume. Alternatively, these cycles
may reflect eustatic change (Mudge & Jones, 2004).

A refined sequence stratigraphic framework was established by Mudge and Bujak
(1996), who used high-gamma value mudstones in combination with
biostratigraphy to subdivide the Paleocene-lowermost Eocene succession
(Rogaland Group plus the Ekofisk Formation of the Chalk Group) into
stratigraphic sequences, high gamma mudstones being interpreted as maximum
flooding surfaces. The stratigraphic position of these high-gamma mudstones
shows a consistent relationship with microfossil bioevents and
biostratigraphic levels (Mudge and Bujak, 1996). These high-gamma mudstones
have been used as the background for the sequence stratigraphic sub division
of the shale formations in this study.

Geographic distribution

The Rogaland Group is continuously present in the sedimentary fill of the
Central Graben, Viking Graben and Sogn Graben. It is also present at the Måløy
terrace, in the Stord Basin and in the Norwegian Danish Basin, but is sub
cropping against younger strata of various ages in the eastern parts of these
areas.
Paleogeographic maps for the Rogaland Group are shown in Fig. 38.

Fig. 38. Distribution of the Formations of the Rogaland Group and its
sandstone members.

Depositional environments

The Rogaland Group was deposited in a bathyal to neritic environment. The
majority of the wells in the Norwegian North Sea have been drilled into
deep-water deposits, with few penetrating shallow-water deposits. Since
exploration has been focused in areas of basin that contain deep-water
deposits, the understanding of these is better than that of the shallow-water
time equivalent deposits.

It was Parker (1975) who first interpreted Paleocene sandstones of the North
Sea to be submarine-fan sediments. It is now widely accepted that the dominant
process for sand transport from the shelfal areas into the various basins and
sub basins were of confined and unconfined gravity flows derived from either
point sources or line sources (Ahmadi et al., 2003; Richards & Reading, 1994).
Depositional features of confined systems involve depositional channels,
sometimes erosional channels, overbank and levee deposits and channelised
lobes. Features of the unconfined systems include single terminal lobes,
amalgamated and compensating lobes and submarine aprons (Ahmadi et al., 2003).
In both confined and unconfined systems, sand deposition and preservation have
been controlled by sea-floor topography. The sedimentation process included
high and low-density sediment gravity flows, slurries, slumps and debris
flows.

Low-density gravity flow deposits are commonly observed in overbank deposits
of levee and crevasse splays, in distal terminal lobes and distal submarine
aprons. They may also be a result of flow stripping from high-density gravity
flows. Slumps are common features in the overbank deposits (Ahmadi et al.,
2003). High-density gravity-flow deposits often occur in confined channels,
proximal terminal lobes and submarine aprons, where they exhibit massive,
unstructured sandstones, massive sandstones with load and dish structures and
planar-laminated sandstones. High sedimentation rates and dipping
morphological gradient frequently led to post-depositional mobilization and
redeposition of sediments seen as debris flow deposits, sandy injectites and
slumps.

Volcanic activity and deposition of tuffs

The volcanic tuffs have already been mentioned in chapter 2. In the North Sea
tuffs were mostly deposited in the Balder Formation, but some minor tuff
stringers are also sometimes seen in the Lista and Sele Formations. These
tuffs are deposits from airborne volcanic ash material settling on the
seafloor. Commonly a bright grey colour is observed in the tuffite
layers/stringers, but may change into pale greenish grey.

By Sand/Gross ratio is meant the relative proportions (cumulative sand
thickness) of sand in a Gross stratigraphic interval, read as a fraction.

Cross plotting of S/G from wells is a technique that has been much used to
predict sand/reservoir presence in Paleocene exploration targets.
By cross plotting S/G data from wells drilled in a certain area, a prediction
of background shale, shale cut-off, sand thickness and S/G ratio of a prospect
can be made.
The technique is most reliable in basinal positions, and must be limited to
sub basins and sub systems since there is much variability between individual
sub basins.

It must be stressed that this technique works best in basinal position with
aggradation, and does normally not give a good correlation in prograding slope
settings.

Fig. 39. Sand/Gross ratio for the Rogaland Group from ~300 wells in the
Norwegian North Sea. From Brunstad 2002.

Fig. 40. Sand/Gross Trends for the Rogaland Group. Trend lines are based on
cross plots of wells in various subareas of the North Sea. From Brunstad
2002.

Fig. 41. Sand/Gross from cross plotting of each of the formations of the
Rogaland Group in southern parts of Quadrant 25. From Brunstad 2002. Crossing
point with the horizontal axsis gives the treshold thickness for sand,
whereas slope of line indicates N/G that can be expected for sandstone bodies
in each of the Formations.

Importance of provenance areas

The major source areas for sands being shed into the North Sea Basin are the
East Shetland platform and south western Fennoscandia. Sandstones vary in
composition from almost pure quartzite to arenaceous sandstone to highly
micaceous or glauconitic sandstone. This is a result of factors such as:

Mudge D. and Bujak J.P., 1996. An integrated stratigraphy for the Paleocene and
Eocene of the North Sea. From: Knox R.W.O'B-, Corfield R.M. and Dunay R.E. (eds.),
Correlation of the Early Paleogene in North West Europe, Gological Society Special
Publication No. 101, pp 91-113.

Mudge D.C. & Jones S.M, 2004. Palaeocene uplift and subsidence events in the
Scotland-Shetland and North Sea region and their relationship to the Iceland Plume.
Journal of the Geological Society, May 2004

Neal J.E., 1996. A summary of Paleogene sequences stratigraphy in North West Europe
and the North Sea. From: Knox R.W.O'B-, Corfield R.M. and Dunay R.E. (eds.),
Correlation of the Early Paleogene in North West Europe, Gological Society Special
Publication No. 101, pp 15-42.

Richie J.D & Hitchen H. 1996. Early Paleocene offshore igneous activity to the
northwest of the UK and its relationship to the North Atlantic Igneous Province. From:
Knox R.W.O'B-, Corfield R.M. and Dunay R.E. (eds.), Correlation of the Early Paleogene
in North West Europe, Geological Society Special Publication No. 101, pp 63-78.